Electronic circuits (hereinafter referred to as “in-vehicle electronic circuits”) obtained by soldering electronic parts such as semiconductor and chip resistor parts to printed circuit boards (hereinafter referred to as “printed boards”) are mounted on a vehicle. Such an in-vehicle electronic circuit is used in a unit for electrically controlling components such as engine, power steering and brake, and such a unit is a safety part which is very important for the vehicle travel. In particular, an in-vehicle electronic circuit unit called ECU (Engine Control Unit) which is provided with an electronic circuit allowing a computer to control the vehicle travel and particularly the engine operation in order to improve fuel efficiency must be operable in a stable state without any failure over a prolonged period of time. In general, the ECU is often disposed in the vicinity of the engine and is in a rather severe operating environment. In this specification, the in-vehicle electronic circuit unit is also referred to simply as “ECU” or “ECU electronic circuit unit.”
When the engine is rotated, the vicinity of the engine where such an in-vehicle electronic circuit is disposed has a very high temperature of 125° C. or more. On the other hand, when the engine rotation is stopped, the outside air temperature in the winter season lowers to −40° C. or less in cold regions such as North America and Siberia. Therefore, the in-vehicle electronic circuit is exposed to heat cycles in a range from −40° C. or less to +125° C. or more by repeatedly operating and stopping the engine.
When the in-vehicle electronic circuit is disposed for a long period of time in an environment in which the temperature thus varies considerably, electronic parts and a printed board cause thermal expansion and contraction. However, since there is a large difference between the coefficient of linear expansion of the electronic parts mainly made of ceramics and that of the printed board made of a glass epoxy substrate, a certain degree of thermal displacement occurs at each of portions where the electronic parts are joined to the printed board by soldering (hereinafter referred to as “solder joint portions”) during the use in the foregoing environment and the solder joint portions are repeatedly stressed by such temperature variations. Then, such stresses eventually cause breakage of the solder joint portions at their joint interfaces. In the electronic circuit, when a solder joint portion is not completely broken but is cracked even at a crack ratio of up to 99%, the circuit resistance value may increase to cause malfunction even in an electric conduction state. The situation that a crack occurs in the solder joint portion to cause malfunction of the in-vehicle electronic circuit unit and in particular the ECU must be avoided. As described above, the temperature cycle characteristics are particularly important for the in-vehicle electronic circuit unit and in particular the ECU, and the solder joint portion, that is, the solder alloy for use therein is also required to be usable even under the severest possible temperature conditions.
A lead-free solder for vehicles (WO 2009/011341A, Patent Literature 1) comprising: 2.8 to 4 wt % of Ag; 1.5 to 6 wt % of Bi; 0.8 to 1.2 wt % of Cu; at least one selected from the group consisting of Ni, Fe and Co in a total amount of 0.005 to 0.05 wt %; and a balance of Sn is disclosed as a solder for use in an in-vehicle electronic circuit unit and in particular an ECU which is under severe conditions of use.
There is also disclosed a soldering material (JP 2006-524572 A, Patent Literature 2) comprising an alloy that comprises, as the simple solder alloy composition, 10 wt % or less of Ag (silver), 10 wt % or less of Bi (bismuth), 10 wt % or less of Sb (antimony) and 3 wt % or less of Cu (copper) in addition to Sn (tin) as the major constituent, wherein the alloy further comprises 1.0 wt % or less of Ni (nickel).